Cool weather slows weed control; spray when warm

Weed control at spring planting, an increasing challenge with herbicide-resistant weeds, presents extra concerns when temperatures stay low, day and night. Weather bears watching more than usual.

Kevin Bradley, University of Missouri weed specialist, knows how to control weeds when warm weather makes plant biology work.

When spring air temperatures rise above 60 F and don't drop to frosty levels at night, the plants, including weeds, grow rapidly.

With normal temperatures the leaf pores open and plant juices flow. That's good for weed control.

Many burndown herbicides applied prior to planting depend on that biology. Sprayed on the leaves, the herbicides "translocate." That is, they move from leaf to root, where the chemistry kills the plant. Weeds are controlled.

Bradley normally urges growers to apply weed control early in the season. That catches weeds just after seedling stage, when they're small and vulnerable. Small weeds require less herbicide.

This spring, weather has not cooperated. Days above 60 F have been rare. And many nights drop well below 40 F. The weed seedlings are there, but their biology is not in overdrive as usual this time of year.

When temperatures drop below 40 F, weed control falters.

Any burndown application, including glyphosate, requires systemic action, Bradley says. The herbicide needs time to penetrate leaf cuticles and move in the plant for optimum effect.

"Weed control will be poor if you make a burndown application and extended cool, cloudy weather follows. That's an extra concern if temperatures drop below 40 F."

Bradley, with help of MU climatologists, looked at April daily temperature cycles across mid-Missouri. Temperatures were lowest 3 to 6 a.m. They reached peaks at 2 to 3 p.m.

Over the last 14 years, records show April temperatures dropped below 40 F as much as a third to half the days, depending on location.

Burndown failures have been common when temperatures drop below 40 F, Bradley adds. "Failure was most likely directly related to air temperature at or after application."

Variations occur among herbicides and among weeds. This makes management difficult. "There have been few studies on temperature impact on control," Bradley says.

He advises farmers ready to apply burndown to watch local forecasts closely.

"Wait for favorable temperatures," he says. "We realize this isn't always possible. Decision must be balanced by size of weeds at time of application. Size may determine decisions. You can't wait so long that weeds exceed the best size for control. This can happen quickly with horseweed and giant ragweed this time of year."

Weed control concerns increase when cover crops were grown on cropland over winter. Cool weather increases risk of failed control. Those beneficial plants become weeds in the next crop.

"If you realize you must spray in cool weather, you may want to increase the rate of glyphosate or whatever burndown herbicide you use."

To improve control, add at least one other tank-mix partner to ensure better success.

Even with low overnight temperatures in the forecast, apply at the time of peak warmth and sunshine in the afternoon. That is when best growth and translocation occur. Avoid early mornings and later afternoons.

"Realize you are not the only one," Bradley says. "I get lots of calls about when to spray in this cool weather."